Recently, content such as music, video or games has been recorded as digital data on media such as CDs, and those media have been widely sold. Digital data undergoes no or little degradation in image quality or the like, and those media in themselves are cheap. Thus, the sale of illegal copies of such content has become a problem.
There has been heretofore proposed a technique which involves prestoring a list of identification information on illegally copied content, and determining whether or not identification information on newly played content is included in the list. See Japanese Unexamined Patent Application Publication No. 2002-135243. According to the technique, a play back apparatus stores the list and adds to the stored list identification information acquired from outside. Thus, the technique enables appropriate restriction upon play back of even illegal copies detected after the manufacture of the play back apparatus.
However, effective prevention of play back of illegally copied content requires that a content play back apparatus should store therein a large amount of identification information on illegally copied content. Thus, the play back apparatus must include a storage device to store the identification information, and this results in a problem of raising the cost to manufacture the play back apparatus.
At the time when a content distributor or the like identifies identification information on illegally copied content, the content may be already out of date. In this case, a problem exists: even restriction of play back of the content has little effect of alerting wicked users because users no longer use the content.